This invention is concerned with wind-driven electric generator apparatus, and more particularly with a downwind, free-yaw, free-pitch machine
A free-yaw machine is very desirable, because it allows rotor blades to track the wind azimuthally and thereby generate electricity more efficiently by virtue of the fact that the blade tips rotate in a plane perpendicular to the wind when the wind direction is horizontal. However, downwind free-yaw machines have never been entirely satisfactory, because there are several conditions in which the blades may strike a support tower and destroy themselves. In general, these conditions are caused by an excessive yaw rate. In some locations, these conditions may occur three or four times a year.
In an effort to prevent an excessive yaw rate in free-yaw machines, yaw dampers have been used, but these devices impose a drag on the yaw movement even when the yaw movement is relatively slow, and machines employing such yaw dampers are not, strictly speaking, free-yaw machines. To avoid the disadvantages of free-yaw machines, yaw drive systems (with a yaw-sensing flag) have been employed to maintain the generator head azimuthally aligned with the wind. Such systems are complicated, expensive, and trouble-prone. Moreover, even with sophisticated yaw drive systems, inaccuracies in the yaw alignment at times causes the wind generator to run very roughly and to vibrate excessively, resulting in excessive stress and wear and tear on the machine.
For maximum efficiency, it is desired that the plane of rotation of the blade tips be maintained perpendicular to the wind direction, and when the wind direction is not horizontal, this requires elevational alignment as well as azimuthal alignment. Conventional machines that are designed to have a rotor axis that is maintained in a horizontal plane cannot take full advantage of the many good wind sites on the upslope of a mountain or hill, where the wind may come up the slope at an angle of 10.degree. to 20.degree. to the horizontal. In such environments the blades will be forced to operate at an angle to the airflow, with resulting decrease in efficiency and loss of power production. Moreover, a machine that is compelled to operate continuously with the blades at an angle to the wind is a very rough-running machine, with high stresses applied to various parts of the machine.
Many years ago it was discovered that a teetering rotor system on a helicopter or gyroplane operated much more smoothly and transmitted lower forces to the rotor-supporting structure of the machine. Modern wind-driven electric generators have used rotors with teetering hubs to provide the same advantages. The most efficient and cost effective wind-driven generators have employed a two-bladed teetering hub with flexible blades, the cone angle of which varies with wind velocity. The higher the wind velocity, the more the flexible blades cone.
When the rotor blades are forced to operate at an angle to the airflow, the blades must teeter about the teetering axis. If the teetering axis is spaced from the center of gravity of the rotor system, vibration and resultant high stresses occur. Due to changes in coning, the center of gravity of the rotor system changes with wind velocity, making it impossible to design a rotor system that always teeters about an axis coincident with the center of gravity of the rotor system. Moreover, teetering hubs are heavy, costly, and expensive to maintain.
While the blades of most modern wind-driven electric generators may be quite flexible with regard to coning, they require considerable edgewise stiffness to ensure that the edgewise natural frequency of the rotor is always greater than the rotational speed of the rotor. The rotor, as it revolves, is influenced by gravity twice per revolution, and if the natural frequency of the rotor is the same as or less than the rotational speed, it is possible for the rotor to become excited by the force of gravity and destroy itself in a few revolutions. Moreover, the natural frequency of the supporting tower must be low enough, or the tower stiff enough, to prevent vibration of the rotor from damaging the tower.
Conventional wisdom in the art has hypothesized that a 400 to 500 ft. diameter rotor may be a practical size limit, because as the blade length increases, the weight of the rotor increases much more rapidly than the edgewise stiffness, and as stated earlier, the edgewise stiffness of the blades must be sufficient to ensure that the edgewise natural frequency of the blades is greater than the rotational speed of the rotor.